A micro-electro-mechanical system (MEMS) technique is advanced and is applied to various fields. In the field of optical communication, a micromirror, for example, to which the MEMS technique is applied is developed and is used in a variable optical attenuator, an optical switch, or the like.
An optical switch includes a micromirror to which the MEMS technique is applied and the angle of a reflecting surface of which can be controlled. Input light inputted from a plurality of input ports is reflected from the micromirror and is inputted to each of output ports selected from among a plurality of output ports. By doing so, optical communication can be performed. Such an optical switch will now be described.
FIG. 20A is a view for describing a light reflection mechanism of an optical switch. FIG. 20B is a view for describing a surface of a collimating lens of the optical switch to which output light is inputted. An optical switch 500 depicted in FIGS. 20A and 20B includes only components that are necessary for describing the light reflection mechanism.
As depicted in FIG. 20A, input light P1 inputted to the optical switch 500 is reflected from a micromirror 510a. Output light Q1 reflected from the micromirror 510a passes through a lens 540 and is inputted to a cylindrical collimating lens 520.
The micromirror 510a of the optical switch 500 is then rotated to the position of a micromirror 510b in order to perform switching of input light. In this case, input light P2 inputted from the same direction where the input light P1 is inputted is reflected from the micromirror 510b. As depicted in FIG. 20A, output light Q2 reflected from the micromirror 510b is outputted at an angle to the output light Q1. The output light Q2 is then condensed by the lens 540 and is inputted to the collimating lens 520.
As depicted in FIG. 20B, compared with the output light Q2 inputted to a portion X2 near an edge of a surface of the collimating lens 520, the output light Q1 inputted to a portion X1 near the center of the surface of the collimating lens 520 is small in attenuation amount. An attenuation amount corresponding to a position on the surface of the collimating lens 520 to which output light is inputted will be described later. The output light Q1 and the output light Q2 pass through the collimating lens 520 and an optical fiber 530 and are outputted to the outside.
FIG. 21 is a graph showing an attenuation amount corresponding to a positional deviation between a position on a surface to which light is inputted and the center of the surface.
As has been described in FIG. 20B, the output light Q1 and the output light Q2 reflected from the micromirrors 510a and 510b, respectively, differ in attenuation amount. This depends on positions on the surface of the collimating lens 520 to which the output light Q1 and the output light Q2 are inputted. FIG. 21 illustrates an attenuation amount of light corresponding to a position to which the light is inputted. In FIG. 21, an x-axis indicates the amount ([mm]) of a positional deviation between a position on the surface of the collimating lens 520 to which light is inputted and the center (point O on the graph) of the surface of the collimating lens 520 and a y-axis indicates an attenuation amount ([dB]) of light corresponding to a position to which the light is inputted with the intensity of the light at the center of the surface of the collimating lens 520 as reference.
According to the graph plotted in FIG. 21, an attenuation amount increases like a quadratic function (square law characteristic) with an increase in the distance between a position on the surface of the collimating lens 520 to which light is inputted and the center of the surface of the collimating lens 520. That is to say, the intensity of light decreases with an increase in the distance between a position on the surface of the collimating lens 520 to which light is inputted and the center of the surface of the collimating lens 520. In addition, an attenuation amount increases like a quadratic function, so the rate of an increase in attenuation amount becomes higher with an increase in the distance between a position on the surface of the collimating lens 520 to which light is inputted and the center of the surface of the collimating lens 520. For example, an increase in attenuation amount caused by the positional deviation from the point O to point a1 is compared with an increase in attenuation amount caused by the positional deviation from point a2 to point a3. In both cases, the amount of the positional deviation is dx [mm]. In the former case, the difference in attenuation amount is dy1 [dB]. In the latter case, however, the difference in attenuation amount is dy2 (>dy1) [dB].
If the optical coupling characteristic of output light is a square law characteristic, then an optical module such an optical switch is apt to be influenced by fluctuations in driving voltage, power supply noise, or external noise. As a result, the intensity of output light is apt to fluctuate.
Accordingly, a method for controlling fluctuations in the intensity of light even under an external influence by changing the optical coupling characteristic of output light from a square law characteristic to a linear characteristic is proposed. For example, output light reflected from a micromirror is made to pass through a transmission filter on which transmittance differs among different positions, and is inputted to a collimating lens. By doing so, the optical coupling characteristic can be changed from a square law characteristic to a linear characteristic and fluctuations in the intensity of light caused by an external influence can be controlled (see, for example, Japanese Laid-open Patent Publication No. 2008-40435).